The invention pertains to a thixotropic agent comprising the reaction product of an alkyd resin and a poly(ester)amide obtained by reacting a polycarboxylic acid with a compound of the formula Xmxe2x80x94Rxe2x80x94Yn, wherein R stands for an organic group having at least 2 carbon atoms, X and Y may be the same or different and stand for a primary or secondary amino group or a hydroxyl group, and m and n are each at least equal to 1, to an alkyd resin containing such a thixotropic agent, and to a coating composition incorporating such an alkyd resin.
A thixotropic agent of the above-mentioned type is known from U.S. Pat. No. 5,591,793.
The thixotropic agent described in said document is obtained by reacting an alkyd resin with a polyamide based on a polycarboxylic acid, for which preferably use is made of an oligomer obtained from soybean oil, castor oil, tall oil, cotton seed oil, corn oil, kapok seed oil, linseed oil or mixtures thereof, and on a compound of the formula Xmxe2x80x94Rxe2x80x94Yn, for which, according to the description and the examples, only aliphatic diamines are eligible, as well as on a monocarboxylic acid for capping any reactive groups still present.
Although the thixotropic agents described in said document give favourable results when used in coatings and topcoat compositions based on alkyd resins, it was found that the thixotropic agents are wanting in some respects. For instance, it is very difficult to carry out the preparation of the known thixotropic agents in a reproducible manner. If the reaction is too short, a cloudy product is obtained. If the reaction is continued too long or performed at a too high temperature, the product obtained, while clear, has insufficient thixotropic properties. Moreover, it was found that there is room for improvement both of these products"" thermal stability at 45xc2x0 C. and of the resistance to polar solvents.
DE 1,234,343 discloses a process for the production of thixotropic coatings from (alkyd) oils with polyamide resins, and GB 1,252,282 discloses a thixotropic vehicle composition. However, neither DE 1,234,343 nor GB 1,252,282 discloses the poly(ester)amide component of the present invention.
U.S. Pat. No. 5,536,871 discloses a liquid rheological additive comprising the reaction product of a liquid polyalkoxylated nitrogen containing compound, a polycarboxylic acid, and a liquid diamine. EP 467,533 discloses rheological additives prepared from carboxylic acids and amines. However, none of these documents discloses the alkyd resin or the poly(ester)amide of the present invention.
The known products based on urea have the drawback with urea it is often difficult to obtain clear products. If clear products are obtained, they often give handling problems and their thixotropic properties are wanting.
To overcome these drawbacks polyurethanes have been suggested. However, polyurethanes have the drawback that comparatively large amounts of them need to be incorporated into a coating composition to achieve a sufficiently thixotropic effect, while being comparatively expensive.
The invention now provides a thixotropic agent for alkyd resins which continues to have high thermal stability at a temperature of at least 45xc2x0 C., which is easily and reproducibly prepared, and which gives few if any problems when it is incorporated into a coating composition.
The invention consists in that in the thixotropic agent of the known type mentioned in the opening paragraph at least 50 mole % of the polycarboxylic acid is a dimeric fatty acid having at least 36 carbon atoms, in at least 25 mole % of the compound of the formula Xmxe2x80x94Rxe2x80x94Yn R stands for a substituted or unsubstituted aromatic group having 6 to 18 carbon atoms and X and Y may be the same or different and stand for a primary or secondary amino group, and in at most 75 mole % of the compound R stands for a substituted or unsubstituted aliphatic group having 2 to 54 carbon atoms, an araliphatic group having 7 to 18 carbon atoms and/or an aromatic or cycloaliphatic group having 6 to 18 carbon atoms.
It should be noted that the possible use of an aromatic diamine in a thixotropic polyamide resin was mentioned in passing in GB-A-915,702. At issue there is always a small quantity in combination with an excess of a primary aliphatic polyamine. On the other hand, EP-B-553 663 (p. 2, line 19) explicitly advises against the use of aromatic amines to prepare a thixotropic agent therefrom.
For that reason it must be considered extremely surprising that it is not only possible to carry out the preparation of the presently proposed thixotropic agents in a readily reproducible manner, but also that the resulting product exhibits remarkably favourable behaviour as regards the handling properties, without this being at the expense of the thermal stability.